The invention relates to a process for the purification of alkali metal or ammonium tungstate or molybdate solutions, which are contaminated with hydrolyzed ions. (particularly of silicon, phosphorus, arsenic, tin, antimony, vanadium, tantalum etc.) formed during alkaline treatment of tungsten- or molybdenum-containing materials for the production of tungstic acid or molybdic acid and for recovery of the alkali content by electrodialysis, wherein the tungstic acid or molybdic acid formed by the dialysis (as the salts of tungsten isopolyacids or molybdenum isopolyacids) is used for acidification of the starting solution.
It is known that alkali (especially sodium) or ammonium tungstate or molybdate solutions formed during alkalinization of tungsten- or molybdenum-containing materials (such as ores) are contaminated with different hydrolyzed ions (silicon, phosphorus, arsenic, etc.). The solutions must be purified before further processing. According to known purification processes, the alkaline solution such as a Na.sub.2 WO.sub.4 solution at a pH of 13 is acidified to a pH of about 9 with an acid such
as H.sub.2 SO.sub.4 or HCl and then aluminum ions are added. The resulting Al(OH).sub.3 precipitate, having a large surface area, binds the impurities present in the solution. The precipitate is removed by filtration and tungstic acid is isolated from the filtrate by conventional methods One such method is by cationic exchange-electrodialysis from the purified alkali or ammonium tungstate solution, whereby the decomposition liquor can also be recovered.
This known process has several disadvantages. On the one hand, there is an economic disadvantage in that a pure acid is necessary for acidification, thus increasing the cost of the process. On the other hand, there are more sources for contamination since sulfate or chloride ions get into the solution.